Embodiments of the present invention describe an antenna arrangement as may exemplarily be used in a transponder reader, such as, for example, an RFID (radio frequency identification) reader or NFC (near field communication) reader. Further embodiments of the present invention describe a transponder reader, such as, for example, an RFID reader or NFC reader.
The use of transponder technologies particularly in medical applications opens valuable possibilities in therapy of human cardiovascular system diseases, for example cardiac insufficiency. Especially for applications where a transponder is coupled with a sensor to be deeply implanted into human body, several requirements have to be kept in mind. These are, for example, a high transmission range and at the same time small transponder antenna dimensions. At present, there are no systems that meet these requirements.
Transponders with attached sensors are of particular interest in medical applications. Implanted inside the human body, such transponders can measure particularly blood pressure and temperature, to improve the therapy of cardiovascular diseases. Especially passive transponders are of interest, because they do not need any power supply in form of a local battery. Thus passive transponders can stay inside the human body for a long time. To make catheter implantation possible, the dimensions of a transponder antenna (loop, coil) are limited to about 2 mm×8 mm (diameter×length).
FIG. 5 shows such a sensor transponder system consisting of a transponder reader 10 and a transponder 20. The transponder reader 10 here is typically located outside the human body so as to read the transponder 20 placed inside the human body. Using its antenna 12, the transponder reader 10 generates a magnetic field which is received by the antenna 22 of the transponder 20 and induces a voltage which is made use of for supplying the transponder 20. The transponder 20 responds to the magnetic field emitted by the transponder reader 10 by so-called load modulation, i.e. by connecting and disconnecting the resistor R of the transponder 20, a current flow IT of the transponder 20 can be changed, thereby loading a magnetic field 23 emitted by the transponder reader 10, which is detected by a receiver 24 of the transponder reader 10 and converted to data.
In addition, electrical losses occur during transmission through body tissues. This fact and the small dimensions of the transponder antenna 22 reduce the so-called mutual inductance between the antenna 22 of the transponder 20 and the antenna of an external device (of the transponder reader 10, usually also referred to as reader) which is able to transmit power supply signals to the transponder 20 and to also receive data transmitted by the transponder 20.
In addition, electrical losses occur during transmission through human tissue. These facts reduce the so-called mutual inductance between the antenna of the transponder and the antenna of an external device (usually called reader) which is able to transmit powering signals to the transponder as well as to receive data which are transmitted by the transponder.
It has to be mentioned that the transmission mechanism used in transponder technology under consideration is typically based on the magnetic coupling of a transmitting antenna loop (e.g. a coil with one or more windings) and a receiving antenna loop (or coil of a transponder, as mentioned) by a magnetic field of the transmitting antenna loop.
This means that the mutual inductance is a measure of the magnetic coupling of the reader and the transponder antenna (loop). In consequence, the maximum possible distance is reduced. For this application in implantable transponders, the bridgeable distance has to be up to 40 cm. To reach this range, optimized antennas and high transmission power are needed. It was shown that a frequency of 6.78 MHz for transmitting powering radiation and receiving data achieves best results in this application. But a frequency of 13.56 MHz achieves acceptable results as well. To provide sufficient energy to the transponder chip over this distance, the voltage amplitude over the reader antenna loop or coil in the relevant system under consideration may be in the range of 240V and more.
Especially in medical applications a sensor transponder should measure a number of physical parameters, such as blood pressure, temperature and also the supply voltage inside the transponder. To make medical diagnostics possible, the pressure history of heart beats has to be transmitted in the resolution needed. Protocol complexity has to be regarded, too.
Moreover, in some sensor transponders parallel measurement and data transmission is not possible because of power limitations. A data rate of 13 kBit/s should be assumed below. It follows that the transmission channel has to have a minimum bandwidth of 26 kHz, using a load-modulation technique.
In some passive transponder systems, load modulation is used to transmit data from the transponder to a reader. Thereby, the impedance of the transponder is changed to modulate a carrier that is produced by the reader. The transponder is provided with energy by this carrier signal. Energy and data transmission are coupled processes. This is one of the disadvantages of this wide-spread technique. To enlarge the energy range, the quality factor of the antennas in the system has to be increased. On the other hand, antennas with high quality factors have low bandwidths. Hence the data rate is limited. Moreover the signal to noise ratio (SNR) and the signal to carrier ratio (SCR) are low in this case. In transponder techniques this problem is called the “quality-bandwidth dilemma”. Especially in medical applications, high transmission range and continuous pressure value transmission (for example of blood pressure) are needed. Calculations in the next section show that these requirements cannot be fulfilled at the same time when using a conventional reader antenna coil.
Analysis of the Transmission Channel
—Transfer Function
The transfer function is needed to analyze the behavior of the transmission channel. For example expectable signal strength and frequency characteristics can be found out. To derive a transfer function the equivalent circuit shown in FIG. 6 is used. It is based on the technical circuit shown in FIG. 5.
The variation of the voltage over the resonant circuit caused by the modulation resistor R can be modulated by a voltage source VT. The transmission channel itself is represented by a transformer equivalent circuit. Losses are represented by RR and RT. The generator is represented by its inner resistance RG. The transfer function VC/VT can be derived by solving Kirchhoff's mesh-law. The result is a first order band-pass function. Using the following parameters the transfer function for this application could be derived. The parameters of a typical arrangement are LR=409 nH, RR=9.8 mΩ, C=1.1 nF, RT=2.4Ω.
The transfer function (based on the assumed parameters) is shown in FIG. 7. By switching the load resistor R, an amplitude variation of the carrier is produced. In the frequency domain, upper- and lower-sidebands appear. The generator signal, transmitted from the reader to the transponder, acts as a carrier for data transmission in the opposite direction.
A 13 kBit/s Manchester coded signal has baseband frequency components at 26 kHz. At the corresponding sideband frequencies, the transfer ratio is about 0.000196. In case of a modulation voltage of 1 V at the transponder side about 200 μV is reached at the reader antenna. In comparison to the sensitivity of common receivers that is about 1 μV, this value is high enough. Sensitivity is no limitation here.
—Signal-to-Carrier Ratio
Usually, transponder systems feature the disadvantage that the received transponder signal is small compared to the transmitted signal, also called carrier signal. This makes signal processing in the reader difficult. High dynamic ranges and low-noise components are needed to enable detection of the transponder. So the so-called signal-to-carrier ratio is of interest to describe the relevant properties of the system. As mentioned before, in such kind of application a voltage amplitude of about 240 V over the reader (=transmitter) antenna coil is needed to provide enough energy to the transponder in a distance of about 40 cm.
Hence the modulation index has a value of:
  m  =                    V        S                    V        C              ≈          8      ,      34      ×              10                  -          9                    ⁢      %      
This value is unfeasible low. To render this value better to detect, a signal-to-carrier ratio (SCR) can be defined by:
  SCR  =            10      ⁢                          ⁢      log      ⁢                        V                      S            -            eff                    2                          V                      C            -            eff                    2                      =                  10        ⁢                                  ⁢        log        ⁢                                            (                              140                ⁢                                                                  ⁢                µV                            )                        2                                              (                              170                ⁢                                                                  ⁢                V                            )                        2                              ≈                        -          121.6                ⁢                                  ⁢        dB            
To make such a signal processable, a sufficient amount of carrier suppression is needed.
—Bandwidth
As mentioned before, in the described example a minimum bandwidth of 26 kHz is needed. By evaluating the transfer function shown in FIG. 7 it can be seen that a baseband bandwidth of about 12 kHz is available here. If the signal was transmitted over such a narrow channel, inter-symbol interference would arise. In this case, a transmitted symbol influences the following symbols during a transmission. This fact makes decoding in the reader difficult or impossible.
—Detuning
Detuning of the reader antenna 12 causes a displacement of the transfer function in the frequency domain. Such a detuning could particularly also be caused by changing the distance between the antennas. As can be seen in FIG. 7, for a higher mutual inductance, the transfer function is shifted to higher frequencies. Because the demodulation is usually done synchronously to the generator signal, the shift also appears in the baseband. In consequence the baseband transfer function is no longer a first order low pass. In other words, the transponder signal is distorted. This effect is noticeable by a beat of the transponder signal. The beat frequency correlates to the detuning.
If the transponder is implanted near to the heart, it will move in rhythm with the heart beat. The mutual inductance depends on the orientation of the coils to each other. Because of that, the damping of the voltage over the reader antenna 12 will vary as well. This variation is noticeable in the baseband signal as a beat which covers the transponder signal.
—Noise
Several noise sources exist in a transponder system. The transponder signal is covered (or disturbed) by these noise voltages and currents. The noise sources are in the frequency generator, the power amplifier, the antenna and in the receiver.
Power amplifier noise consists of shot noise caused by PN junctions and Johnson-noise caused by resistors. Thus, the mean square voltage at the receiver is of interest. The mean square voltage produced by a conventional power amplifier (typical noise figure 16 dB) was measured as 2.3 mV. The gain of the parallel resonant antenna circuit causes an amplification of noise near to the resonant frequency.
With a receiver bandwidth of e.g. 100 kHz, which is needed for a 13 kBit/s Manchester coded signal, a mean square noise voltage of about 115 mV is reached. This value is several orders of magnitude higher than the transponder signal voltage, which is about 200 μV.
One could say that the power amplifier is the dominating noise source in the system and determines the SNR.
The SNR is a measure that describes the quality of the signal. If conventional load modulation is used, it can be calculated as follows:
  SNR  =            10      ⁢                          ⁢      log      ⁢                        V                      S            -            eff                                    V          noise                      =                  10        ⁢                                  ⁢        log        ⁢                                            (                              141                ⁢                                                                  ⁢                µV                            )                        2                                              (                              115                ⁢                                                                  ⁢                mV                            )                        2                              =                        -          58.2                ⁢                                  ⁢        dB            
For the described example a Manchester coded signal with 13 kBit/s is assumed. Usually an SNR of about +10 dB after filtering is needed to get an acceptable BER (bit error rate). This means that data transmission with conventional known antenna coils is not possible here.
Conventional Carrier Suppression Methods
Some known solutions try to overcome this problem. The three most frequently used solutions are presented as follows. However, it will be shown that all these solutions are unsuitable for an application over wider distances (as supposed in the examples).
The existing solutions can be divided into two groups:                techniques using carrier suppression circuit design, and        techniques using special antenna coil arrangementto achieve carrier suppression.        
One possible circuit design solution is using ceramic filters. Like shown in FIG. 8 one side-band 25 could be filtered and another side-band 27 be transmitted. The carrier signal 26 which is located outside the pass-band of the filter is suppressed. This method has several disadvantages. The dynamic range of active elements, like amplifiers, is usually not high enough. Hence, this solution could not be used alone. Only the side-band 27 can be used. Thus, half of the signal energy is lost. Further the bandwidth is still limited by the reader antenna coil. A sub-carrier would enlarge the spectral distance between the carrier 26 and the side-bands 25, 27 and implicate a better SNR. In this case, the data signal is multiplied by a periodic square wave function of constant frequency. Filtering would than be easier. Because of the bandwidth limitation this technique cannot be used in applications considered here (load modulation and medical applications of high data rates).
Beside circuit design solutions, it is possible to realize carrier suppression with the help of a special antenna (loop or coil) arrangement. To make this possible, a separate receiving antenna coil is used in the reader. Now the reader antenna arrangement consists of at least two loops or coils: at least one transmitting loop 12 or coil for energy transmission to the transponder and at least one receiving loop 31 or coil to receive the transponder signal. One possibility to improve the SCR and SNR is a spatial separation of transmitting 12 and receiving 31 coil(s). For example is it possible to place the receiving coil 31 on an opposite position to the transmitting coil 12 as shown in FIG. 9. The SCR is improved because the receiving coil 31 has less distance to the transponder coil 22 than to the transmitting coil 12. Spatial separation causes smaller coupling between the receiver coil 31 and the transmitting coil 12. However, this antenna arrangement is not feasible in most applications.
Additionally, DE 102008034001 A1 shows a pair of coils, including carrier suppression, which comprise a transmission coil and a reception coil. The transmission coil is configured to transmit a transmission signal which comprises a carrier. The reception coil is configured to receive a reception signal which comprises the carrier and data from a source and to suppress the carrier considerably and thus maintain coupling to the source in any position along and any position in proximity to the reception coil.